Page images
PDF
EPUB

JOURNAL OF RESEARCH of the National Bureau of Standards
Vol. 85, No. 1, January-February 1980

Standardization of Iridium-192 Gamma-Ray Sources
in Terms of Exposure

T. P. Loftus

Center for Radiation Research, National Bureau of Standards, Washington, D.C. 20234

September 29, 1979

Iridium-192, in the form of small platinum-or stainless-steel-clad seeds, is used for radiation therapy. Standardization of this radionuclide, for the quantity exposure was carried out by measuring groups of seeds in an open-air geometry, using the NBS standard graphite cavity ionization chambers, and transferring the exposure data to a re-entrant ionization chamber.

Tables are provided from which the corrections for the graphite chamber have been calculated along with corrections for room scattering.

Radiographs of the source arrays are shown and details of the re-entrant chamber source measurements and construction are provided.

As assessment of the errors involved in establishing this standard leads to a statement of 2 percent for the overall uncertainty in the calibration of an iridium seed for the quantity exposure.

Key words: Exposure standard; iridium-192 seeds; NBS standard graphite chambers; open-air geometry; re-
entrant chamber.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small]
[blocks in formation]

The current per unit volume of air, produced by an individual iridium-192 source of 1.5 mCi activity, distant 1 m from an ionization chamber, is about 7 × 10-17 A cm3. This is at least an order of magnitude less than the inherent leakage current for good electrometers. For accurate current measurements, therefore it is necessary to optimize the measurement conditions. This can be done by increasing the activity (by measuring a group of sources), using the largest-volume ionization chamber that is suitable, and using short measurement distances. The constraints imposed by the largest suitable chamber volume (approximately 50 cm3), and a reluctance to reduce the source-to-chamber distance to less than about one-half meter, led to the fabrication of two plane source arrays consisting of large numbers of seeds. The seeds were held in shallow recesses (1 cm by 1

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small]

where X is the exposure3, Qair is the measured charge,: the chamber volume, and g is the density of air at the ti of measurement. The ratio of the mean collision mass sto ping powers for electrons in the medium (in this case grap ite) and in the gas (air), (5/g)air, must be calculated with c sideration of the electron spectra produced by the ma photon energies of iridium-192, as well as for the Spenc Attix parameter ▲, which is the upper limit for electr energy losses and the lower limit on electron energies

The Rad-Irid Co. supplied 49 seeds. However, the seeds were magnetized and could not be to lie in an orderly plane array. Most of the magnetism was erased using a computer tape eras some residual magnetism limited to 46 the number of seeds that could be readily placed

recess.

'The SI units for X are coulombs per kilogram of air. To convert to roentgens divide by 25 |

10 .

cluded in the spectrum. The value of ▲ for the 50 cm3 standard chamber is taken to be 50 keV.

The mean stopping powers, for carbon and air, were determined by computing the initial Compton electron spectra [5] produced by each of the iridium-192 gamma-ray energies, weighting and summing the spectra in accord with "the relative numbers of photons at each energy per disintegration, and then weighting and summing the stopping powers over the resultant electron energy spectrum. Stopping powers for a range of electron energies are given in table 3. The ratio of the weighted mean stopping powers for carbon and air is 1.015.

TABLE 3. Electron stopping powers calculated using the expression of Berger and Seltzer [6] for the restricted mean collision loss, with ▲ = 50 keV, Ic = 78 eV and I = 86.8 eV. The data for carbon are corrected for the density effect, which is assumed negligible for air.

Restricted collission mass stopping power (S/g)

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small]
[blocks in formation]
[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

1.650

1.646

1.002

[blocks in formation]

Other than the experimental corrections included in the product Пk, which are discussed later, the remaining quantities to be determined are the mass energy-absorption coefficients (/) for air and carbon. The data of Hubbell were used to determine the ratios of these coefficients which were then plotted against photon energy. Again, weighting factors (with consideration for attenuation in the seed wall) were used to compute values for Men/Q)air/en/Q)c of 1.001 and 1.002, for the photon energies from the platinum and stainless steel encapsulated sources respectively.

4. Experimental Corrections

The corrections forming the product Пk, are of two kinds: those relating to the particular standard graphite ionization

The standard chamber wall-absorption correction was determined by positioning the plane sources in a collimatorshutter system to allow measurements with different standard chambers, and also to allow the addition of shells to the chambers. This arrangement avoided the need to move the source for protection purposes. The two techniques provided data that, when extrapolated to zero wall thickness, give wall corrections which agree to 0.1 percent. The average values of these corrections for the platinum-clad and steel-clad seeds are given in table 4.

The exposure rates encountered in these measurements are relatively low and only moderately high collection potentials for the ionization chambers are required to minimize recombination effects. Calculations for the 50 cm3 standard chamber, based on previous data [4] for a collection potential of 500 V, predict a correction factor of 1.0013 for the expected exposure rates. Checks were made on the chamber saturation conditions during the open-air geometry measurements by reducing the potential from 500 V to 250 V. Results show a reduction in ionization current of 0.17 percent when the exposure rate is 9.3 nA /kg (36 μR's).

The standard chamber stem-scatter correction was determined by placing a dummy stem on the opposite side of the chamber stem in the collimated beam. The beam was large

enough in diameter so as to irradiate the system uniformly but not so large as to irradiate the supporting stands. This mirror-image experiment provides a stem scatter correction of 0.999.

All standard chamber ionization current data are corrected for atmospheric effects in accord with the ideal gas laws. The influence of water vapor on the measurements as it may affect the air density and the stopping-power ratios has not been taken into account and the correction for humidity is taken to be unity.

Source measurements in an open-air geometry require corrections for scattering from the room surfaces to make the measurement independent of the surroundings, and for air attenuation and scattering to make the source measure

ments independent of source-detector distance. The magnitude of these corrections depends on the various source-towall-to-chamber distances and source-to-chamber distances.

The dimensions of the measurement room are 6.0 m by 9.4 m by 4.4 m high. The source-chamber center line was vertical and roughly at the center of the room. Using the method of Eisenhauer [7], the scattering contribution from each surface of the room was calculated based on an albedo measurement. The measurements show that the concrete floor increases the chamber reading by about 16 percent, when the source is on the concrete floor, over that measured when the source and chamber are suspended in the center of the room with the source-chamber distance being one meter in each case.

Two source-to-chamber distances were used for the measurements, 0.5 m and 1.0 m. The room surface scattering correction factors, k,,, for the 1.0-m and 0.5-m measurements, are calculated to be 0.972 and 0.993 respectively.

The remaining correction is the air attenuation-scattering correction, the work of Eisenhauer [8] being used for the computations. The corrections, kas, for a photon energy range appropriate to iridium-192 and for 0.5-m and 1.0-m distances are given in table 5. Mean values for kas, weighted

TABLE 5. Air attenuation-scattering correction factors, k... in the photon energy range 0.2-0.5 MeV for two source-to-chamber distances.

of source-detector distance and surface-source-chamber dis tances, the measurements are multiplied by the k., and k. factors. In this way, the consistency of measurements at dif ferent distances can be tested and the calibration made independent of the environment. However, when a calibrated source is used in the field, one wants to know the exposure rate at some distance from the source. Then the calibration for the source must be divided by the appropriate attenuation-scattering factor as well as the appropriate surfacescattering factor.

5. Source Measurements

The platinum-encapsulated seeds were measured in the open-air geometry at different times and different dis tances. Although nine measurements were performed, only five can be considered independent-three at approximately 0.5 m and two at approximately 1.0 m from the source. Al ionization current data were corrected for decay using a half-life of 74 days (λ = 6.5 × 106 min1) [2]. Two measurement systems were employed, a vibrating-reed-electrometer null method and a high-gain direct-coupled electrometer with a digital voltmeter for measurement of feedback poten tials. The latter system allows automatic control of the digital voltmeter for initial and final measurements over preset charge-integration intervals. The built-in feedback capacitor in the high-gain electrometer was replaced with a calibrated air capacitor. The agreement among the various measurements is shown in table 6. The consistency of the measurements when corrected for scattering is demonstrat ed by the ratio of (ID2 krs kas) for 0.5 and 1.0 m which is 1.005, with the corrections and measurements data taken from tables 4 and 6, respectively.

[blocks in formation]

kas

[blocks in formation]
[blocks in formation]
[blocks in formation]
[blocks in formation]

to be 2.966 × 10-15 A·cm3 for the array of platinum-encapsulated sources at a distance of one meter.

Using eq (2) for X and the data taken from tables 4 and 6, the exposure rate at one meter from the platinum-encapsulated source array was calculated to be 2.411 n·kg ̄1·m2 (9.346 μR sm2) on the reference date.

The stainless-steel-encapsulated source array was measured with the standard graphite chamber in the open-air geometry on two consecutive days; however, at only one source-to-chamber distance, about 44 cm. With correction for decay and adjustment to a common source-chamber distance, the difference between the two measurements was 0.4 percent. The exposure rate at one meter was 0.5614 nA kg m2 (2.176 μR s1 m2) on the reference date.

[ocr errors]
[ocr errors]
[blocks in formation]

standard deviation of 0.1 percent, based on four measurements with charge integrating times of about 30 s.

The current produced by a 1.5-mCi iridium-192 seed in the aluminum sphere was 55.5 pA, while the same seed in the conducting plastic sphere, of essentially the same volume, produced 49.0 pA. The 13 percent increase in current is due to the energy dependence of chamber response when an air-equivalent material is replaced by aluminum as the wall of the ionization chamber.

The collection electrode is a hollow cylinder surrounding the re-entrant tube. It is guarded and the high voltage insulator is recessed. The chamber is usually operated at a potential of 1100 V on the aluminum sphere, with a positive ground. Since the sphere is at high potential it is necessary to use an insulating holder for sources placed in the chamber. Glass vials of length 65 mm, with an outer diameter of 9.2 mm and wall thickness of 1.4 mm (0.29 g/cm2) are found to be convenient for this purpose. A thin plastic tube is used to insert the vials into the chamber. The internal diameter of the vials is such that the 3 mm-long iridium-192 sources lie almost horizontally at the bottom of the vials.

The recombination characteristics for the aluminum sphere, assuming a linear response proportional to the inverse of the collection potential, are such that the correction for ionization loss is 0.36 percent for currents of 6.6 pA to 76 pA at the operating collection potential of 1100 volts. The percentage loss increases to 0.6 percent for a current of 884 pA for the same collection potential. In these tests, the lowest current was produced by an iridium-192 source while the two higher currents were produced by cesium-137

sources.

The polarity dependence of the chamber is characteristic of the collection electrode-insulator system and was found to be the same for the plastic and aluminum spheres. The ratio of the chamber currents measured with positive ground, to currents measured with negative ground, is 1.0014.

7. Iridium-192 Seed Measurements in Re-Entrant Chamber

Although the plastic re-entrant chamber was replaced with the aluminum chamber, and it will not be used routinely for calibrations, it is of interest to determine the degree of agreement between relative measurements of the seeds, in each chamber, for different measurement conditions. The comparison can be made through measurements of the 53 platinum-encapsulated seeds which were carried out in each of the chambers.

Each of the seeds was placed in a glass vial and measured in each of the re-entrant chambers. The measurements using the plastic sphere were carried out in eight hours while the measurements using the aluminum sphere were com

« PreviousContinue »